The production of dry solvents from raw aqueous mixtures is often costly and complicated. The preparation of dry ethanol is a good example. In the conventional process, the raw fermentation broth is stripped under moderate vacuum in a beer still. Overhead vapor from the beer still is sent to a rectification column that produces an overhead product close to the azeotrope (about 93 wt % ethanol) and a bottoms product, which is essentially water. The condensed product from the top of the column is evaporated under pressure and fed to a molecular sieve dryer, which produces ethanol of 99 wt %+ purity. Such a process consumes almost 100 million Btu/h to produce 50 million gallons per year of purified ethanol from a feed containing about 11 wt % ethanol.
It is known to use membrane separation for dehydration. Such processes are described in co-owned and copending patent application Ser. Nos. 11/715,245 and 11/897,675, for example.
It is also known to use membrane separation to treat the overhead stream from a column. Co-owned and copending Ser. No. 11/494,900 teaches a process for recovering ethanol involving membrane separation, followed by dephlegmation, followed by a second membrane separation step to dehydrate the overhead stream from the dephlegmator.
U.S. Published Application number 2006/0070867 teaches a combined distillation/membrane separation process in which the overhead vapor from the column is condensed, then a portion of the vapor is reboiled before passing as feed to the membrane separation step.
Japanese Published Application number JP7227517 teaches the use of a membrane separation step to treat the overhead from a stripping column, with the permeate from the membrane separation step being returned to the stripping column.
There remains a need for a process that is both energy efficient and cost effective for producing high purity dehydrated solvents, especially ethanol.